The practice of fruit thinning is a standard management procedure for the deciduous fruit industry. Thinning is the removal, or prevention, of a part of the fruit crop before it matures on the tree. The fruit itself may be targeted, or the blossoms which produce the fruit may be the target of the thinning procedure. Fruit thinning is necessary since many fruit trees tend to produce far too many blossoms each year. The excess of fruit blossoms and fruit often results in produce which is too small to be marketable. Further, when the fruit density is too great on portions of the fruit trees, the weight of the fruit may become too large to be supported by the tree, and the entire branch, including its fruit, may be lost. Additionally, fruiting is an exhaustive process to the tree, especially if the crop is heavy. Therefore, one advantage to fruit thinning is to permit the tree to mature as large a crop as possible and yet converse sufficient nutrients and carbohydrates for good shoot and spur growth, leaf development, and flower-bud formation for next year's crop. If the tree is permitted to mature an excessive crop, it becomes devitalized to the point where it not only produces an inferior product, but it becomes increasingly susceptible to disease and low-temperature injury.
An additional problem pertains to certain types of fruit trees which have substantially reduced fruit yields in alternate years. These biennial fruits have excessively large crops followed by insufficient crops in alternate years. This pattern can be disrupted, and constant desirably-sized crops can be obtained on a regular basis by proper thinning techniques. Apples are a very good example of biennial fruits which respond favorably to appropriate thinning.
There are three commonly-used methods for thinning fruit: mechanical, chemical, and by hand. Each has its advantages and disadvantages. In practice, fruit thinning is generally done by hand. Workers must go to each tree and remove sufficient fruit to provide satisfactory fruit thinning. Hand labor is perhaps the most reliable, but it is expensive.
Chemical thinning would be preferred from a cost standpoint, but environmental concerns, as well as inconsistent results, are major drawbacks to existing chemical methods. The inconsistent results include overthinning, underthinning, and foliage injury. The chemical thinning procedures which have been used in the past, and which are currently in use, function through a variety of mechanisms. Some chemicals are applied during bloom and work by burning the pistil to prevent pollination. Other chemicals are applied during various stages of early fruit development to induce fruit drop (Yoskikua and Johnson [1989] Chapter 10 In: LaRue, J. H., R. S. Johnson, eds. Peaches, Plums, and Nectarines-Growing and Handling for Fresh Market, University of California Coop. Ext. Serv. Publication #3331). The success of chemical fruit thinning strategies can depend upon a number of factors including: variety and strain, tree condition, fruit set, proximity to pollinizers, weather, the chemical used, and the specific application protocol.
Among the early chemicals which were used for fruit thinning were tar-oil distillates such as phenols. These compounds were typically sprayed on the trees a day or two prior to full bloom. In the 1960s, several post-bloom thinners were introduced, including methyl carbamates (Sevin 50% from Rhone-Poulenc). One method of chemical thinning uses growth regulators to reduce the amount of fruit and to overcome alternate year or biennial bearing. Certain alkoxylated amines have been described as useful for thinning stone fruit blossoms. See, for example, EPO 463 241. Some growers may use several different chemicals, or combinations of chemicals, during the bloom and postbloom period.
Several chemicals have been or are currently being used in the industry for fruit thinning purposes. Some of these existing products are toxic to both the surrounding environment as well as the operators that are charged with the task of applying these chemical agents. Furthermore, many of the existing products are also phytotoxic to the orchard.
One chemical thinning agent is DNBP (Gebutox.TM. 50% from Hoechst). This agent effectively reduces the number of blossoms on stone fruit trees and leads to increased fruit size. However, the agent has been found to be phytotoxic to the fruit trees and other plants as well. Further, the application of this agent poses some risk to the person applying it. DNOC (Elgetol.TM. from FMC) was a major product used for apple thinning. However, this product has been removed from the market because of toxic properties.
Another agent which has been used for fruit thinning is Paraquat. However, this agent has been found to be toxic to humans and to cause damage to the crop treated as well as other surrounding plants. Both DNBP and Paraquat are now withdrawn from governmental approval in many countries for use in blossom thinning of stone fruit for reasons of toxicological concern and orchard safety.
Currently, the only thinning agents available to growers are NAA (Fruitone.RTM.N from Rhone-Poulenc), ethephon, and NAD (Amid-Thin.RTM.W from Rhone-Poulenc). These agents are growth regulator type materials and provide inconsistent results. Growers can also use the insecticide Sevin, but this material kills the bee population needed for pollination. Accordingly, there exists a need in the fruit industry for a chemical blossom thinning agent which exhibits a relatively low phytotoxicity, is not harmful to humans applying the agent, does not damage pollinator populations, and is capable of providing acceptable and consistent results in blossom thinning and the subsequently required fruit thinning effect. These and other objects of the present invention will be apparent from the summary and detailed description that follow.
Fatty acids are a class of natural compounds which occur abundantly in nature and which have interesting and valuable biological activities. The in vitro activity of fatty acids against many medically important fungi and bacteria is well known. Ahmed et al. ([1985] JAOCS 62:1578-1580) report in vitro inhibition of radial growth of several fungal genera with plant pathogenic representatives. Recently there has been an expanding use of "insecticidal soaps" in agriculture which are salts of certain fatty acids. This has resulted in a few observations of impact on fungal disease. For instance, Chase et al. ([1989] Plant Disease 67:1021-1023) observed that applications of a c18:1 fatty acid salt "insecticidal soap" gave moderate preventive control of two foliage plant diseases and actually exacerbated two other diseases.
Recently, salts of fatty acids, primarily sodium or potassium fatty acid salts, have been used commercially as insecticides. Compositions having excellent insecticidal properties which exploit these salts are available commercially from Mycogen Corporation, under the trademark M-PEDE. A herbicidally active composition utilizing partially saponified fatty acids as the active ingredient is sold by Mycogen under the trademark SHARPSHOOTER. These fatty acid compositions are effective, naturally-occurring pesticides which have no known long-term environmental effects.
U.S. Pat. Nos. 2,626,862; 4,975,110; and 5,035,741 describe certain fatty acid compositions useful as herbicides. These patents also teach that the proper formulation of a fatty acid herbicide requires one or more surfactants. Also, fatty acid esters have been used as chemical pinching agents for the inhibition of bud growth of certain plants (Tso, T. C., G. L. Steffens, M. E. Engelhaupt [1965] J. Agr. Food Chem. 13(1):78-81; Sill, L. Z., P. V. Nelson [1970] J. Amer. Soc. Hort. Sci. 95(3):270-273). There are no reports in the literature on the use of fatty acids or their salts, esters, or other derivatives as fruit thinning agents.